It is often necessary to transfer an accurate volume of a substance from a storage chamber containing that substance to another location, but it is not always easy to effect the transfer accurately and efficiently. In particular, it is often desired to transfer a powdered medicament from a storage chamber to an inhalation passage in a dry powder inhaler.
Powder inhalers, which deliver a drug in a dry, finely divided form, have been shown to give certain medical advantages over other forms of delivery system. In particular, they are more popular than inhalers which propel a drug in pressurised gas from an aerosol, because of environmental and other considerations.
EP-079478, EP-166294 and GB-2165159 disclose examples of dry powder inhalers which include a medicament storage chamber and an inhalation passage through which air is drawn via a mouthpiece. A metering member provided with a metering recess takes a dose of medicament from the storage chamber and deposits it in the inhalation passage.
It is considered that the accuracy of such arrangements can be very poor: on the one hand, by repeated indexing of the metering member it is possible to deposit two or more doses of medicament into the inhalation passage, resulting in the administration to the user of an overdose of medicament; on the other hand, since the medicament normally drops from the metering recess into the inhalation passage under gravity, particles of medicament can adhere to the interior of the metering recess so that an underdose is delivered.
U.S. Pat. No. 2,587,215 discloses dry powder inhalers with the same disadvantages as those mentioned above. However, this document also discloses an embodiment in which the metering member presents the medicament in an upwardly open dispensing cup to a mixing chamber, where it is mixed with air before being sucked into an inhalation tube via a nozzle having a narrow opening. Air sucked into the inhaler passes into the inhalation tube either directly or through the mixing chamber and nozzle. Accordingly, not all the air passes over the dispensing cup. If medicament adheres to the surface of the dispensing cup but is not sucked therefrom, an underdose of medicament will be delivered to the user. Upon repeated use of a dispensing cup to deliver doses to the inhalation tube, a continually increasing amount of the medicament powder can adhere to the base of the dispensing cup, resulting in progressively decreased dosages to the user.
This build-up of adherent powder is thought to be a source of inaccurate dosing in many of the inhalers previously proposed.
Moreover, in the inhaler of U.S. Pat. No. 2,587,215, the metering member is a rotary sliding device journalised on a cylindrical pivot member extending from the bottom of the body of the device. Such an arrangement is susceptible to jamming due to ingress of powdered medicament between the cylindrical contacting surfaces of the pivot member and the metering member.
Another form of inhaler which is currently available includes a metering member having a number of tapered metering recesses which are open at top and bottom. In use, finely divided medicament from a storage chamber is packed into the recesses, whereupon the metering member is moved to a dispensing position in which air can be drawn through the recesses to draw out the medicament. This device is considered to have a number of major shortcomings. Firstly, the metering recesses are prone to clogging. Secondly, a large amount of suction is required, so that the device is unsuitable for use by patients with breathing problems. Thirdly, two hands are required to operate the device.
An improvement over the above described inhalers is described in our earlier PCT patent application, No. PCT/GB91/01147. This discloses an inhaler in which the metering member comprises at least one dispensing cup which, when filled from a storage chamber with a dose of the substance to be delivered, is presented to the inhalation passage in an upwardly open position. The substance to be delivered is removed from the dispensing cup, rather than by the action of gravity, by air flow through the inhalation passage. Thus, in normal use of the inhaler, repeated indexing of the metering member should not result in multiple doses of the substance being delivered into the inhalation passage.
The inhaler described in PCT/GB91/01147 incorporates means for ensuring that the dispensing cup is substantially free from the substance to be delivered, before it is presented to the storage chamber to be re-filled with the substance. This means may comprise a specially shaped inhalation passage; means for moving the dispensing cup into a downwardly open position after it has been presented to the inhalation passage and before it is re-presented to the storage chamber; or means, such as resilient wiping means, for dislodging any remaining substance from the cup after it has been presented to the inhalation passage. In all cases, at least one of these special means must be included in the inhaler so as to prevent any of the substance from remaining in the dispensing cup after it has been presented to the inhalation passage. If any of the substance did remain, this could affect accuracy when the dispensing cup was refilled from the storage chamber ready for subsequent representation to the inhalation passage.
The need to ensure that a metering member is substantially free from the substance to be delivered, after each presentation to the inhalation passage of an inhaler, arises largely from the fact that the metering member is constantly being re-presented to the storage chamber, re-filled with the substance and returned to the inhalation passage. The metering member is required to deliver an accurate dose of the substance each time it passes through the inhalation passage.
It is an aim of the present invention to provide improved means for accurately transferring a dose of a substance, such as a powdered drug, from one location to another, which means may be used, inter alia, in an inhaler to transfer a dose of drug from a storage chamber to an inhalation passage. An inhaler incorporating such means should overcome or at least mitigate the above described problems associated with conventional inhalers, and should be capable of delivering an accurate dose of drug to a user each time it is used.